The present invention relates to a method for generating mutually orthogonal signals having a controlled spectrum.
It has numerous and varied fields of application, including tattooing files of signals such as audio signals.
Audio tattooing by spectrum spreading uses temporal signals with a wide spectrum referred to as an extended spectrum. Tattooing uses either one or a number of signals stored in a dictionary for the modulation of symbols. When a plurality of signals is used, it is preferable to use signals whose intercorrelation product is zero, as this facilitates detection by correlation. The intercorrelation of the signals is a particular form of the scalar product. It can thus be said that seeking signals that are not correlated with each other amounts to choosing a family of mutually orthogonal signals.
In the context of tattooing, there is a requirement for signals with a controlled spectrum, i.e. that correspond to a particular template. For example, a signal coded using advanced audio coding (AAC) at 24 kbps per channel occupies a band of the order of 7 kHz, whence the benefit of controlling the spectrum of the mark by limiting its bandwidth to 7 kHz. Moreover, tattooing must also be as discreet as possible, and will therefore be modulated and shaped taking account of psychoacoustic properties. To guarantee accurate shaping, below the masking curve guaranteeing the inaudibility of the mark, the signal to be modulated must have a perfectly blank spectrum.
Real mutually orthogonal temporal signals of given length are generally generated either by orthogonalizing a family of temporal, generally random, signals or by using lines or columns of real Hadamard matrices.
Each of these prior art techniques has the drawback of producing signals whose power spectrum is difficult to control. For orthogonalization, for example, dynamic range variations of 60 dB and more are observed over the power spectrum, the spectrum being chopped and highly irregular. Moreover, the real Hadamard matrix technique can produce only signals of length 2 or multiples of 4.
In code division multiple access systems, for example CDMA systems conforming to International Standard 95, the signals used to separate the users are real orthogonal Hadamard sequences with spectrum that depends on the sequence number. This approach is described in “UMTS, les origines, l'architecture, la norme”, Pierre LESCUYER, Dunod, 2nd edition, 2002. For example, the first line of the Hadamard matrix contains only 1s. As a result this step must be followed by a step of spectrum spreading as such, or scrambling, before modulation (see pages 116 to 119 of the above work), which increases the complexity of this kind of system.
The document WO-A-00 77962 describes a method for generating small numbers of complex orthogonal spectra, typically seven spectra, by discretizing the phase of each complex sample. This method has the particular drawback of providing only a restricted number of sequences.